I have a Nikon DX (D5100) camera with the two most basic kit lenses: 18-55mm f/3.5-5.6 VR and 55-200mm f/4-5.6 VR. These two lenses cover lots of scenarios, but I'm starting to believe there is more that awaits me. I'm only a hobbyist, so I'm not sure what exactly :). First of all, I don't want to spend a lot of $. So I started checking out the cheap 35mm f/1.8 prime lens. Reviews say that, with the same exposure settings, the image quality of this lens is superior to mine. However, I started doing the math and got confused.

First of all, the advertisement says that VR effectively gives me +3 f-stops. Am I correct in assuming that the +3 f-stop advantage of VR means that eg. a 1/30s exposure will be as stable as one with 1/240s without VR, and it doesn't change anything else? But I guess it has its limits, so I doubt that it can make anything below 1/30s stable.

So let's compare my lens with the prime lens. Let's say that I set my wide zoom lens to 35mm, max aperture is around f/4.5, VR is off, and let's say that this is too slow for me because it's too dark for example. I can turn on VR, I get +3 f-stops. The 35mm prime lens is f/1.8, which is 2.666 f-stops away from f/4.5. It's not that better, or is it?

I know that advertisements can exaggerate, but I trust Nikon that VR is not totally useless at least. So if my math is correct, then the prime lens is not all that better exposure-wise (except for the supposed image quality). What do you think VR actually gives me and what does the bigger aperture prime lens give me?

5 Answers
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But I guess it has its limits, so I doubt that it can make anything below 1/30s stable.

I think that is not correct and you can go below 1/30s. This depends on focal length of course.

What will the 35mm f/1.8 give you:

Shallow Depth of Field: It will give you nice background blur if you want it. However, a 50mm f/1.8 with the same framing of an object will give better blurred background (because DoF depends on focus distance, too).

2 stops of more light coming in. This allows faster shutter speeds, so you can freeze motion more easily. However, if your subject moves you still have to focus and that doesn't change between these lenses (more or less).

Prime lenses are generally of better optical quality. This is also true for the 35mm compared to the 18-55 that you have.

Ask yourself, what you want to use it for. It is great for upper-body portraits. Night street photography, and many other things but certainly not for all things.

Image stabilization will prevent 3 stops of camera shake, but it does nothing for subject movement. You can take photos by hand at much, much slower shutter speeds with image stabilization, but the movement of subjects will be a problem.

Keep in mind there is also far more to lens speed than shutter speed. You also have the depth of field impacts of being able to open the aperture further. This can be a strength or a weakness depending on the situation. If you want to take a photo with a big depth of field and an unmoving subject, the stabilized lens will serve you better as it can get the shot at a lower aperture. On the other hand, if you need to shoot a moving target or want the highest degree of background blur possible, then the fast lens is the only way to go.

The optics in a prime are also much simpler and thus the image quality is generally better than a similarly priced zoom lens. Comparing apples to apples (same focal length and aperture), you'll probably still notice a slight improvement, but it will be countered somewhat by the gain that the image stabilization provides. If you use a tripod, the prime will have a clear advantage however.

A better comparison is really the slower lens with VR on at minimum f/number and the faster lens at it's max f/number. This is the real world apples to apples comparison for low light since you would shoot wide open for both and in decent light, the shutter speed is much less of a factor.

Your observations also seem to be fairly true across lens manufacturers for similar situations. I did a similar test with a stabilized Canon lens that was f/4 and compared it to an f/2.8 that was $700 more (both were zoom lenses). In that case, when shooting the same f/4 on both, there was fairly minimal quality difference, but when shooting freehand at f/2.8 and f/4, it became more noticeable and when comparing the background blur in a well lit shot, the added sharpness and the shortened depth of field became major factors in the quality of the lens. (To the point that the IQ of the f/2.8 was probably roughly double the f/4 with less than a 50% difference in price. $1500 vs $2200)

Prime lenses usually have an image quality far superior than zoom lenses for a cheaper price.

Image quality means the following (among others):

sharpness

micro contrast

colour saturation

chromatic aberrations

geometric aberrations

bokeh!

And then it's the speed, which is not only important for the ability to shoot handheld in low light situations, but also to freeze the action and give a shallow depth of field.

Also, they are smaller and lighter, so you can carry 2 or 3 really nice glass for the same weight and size of a constant f2.8 lens.

But prime lenses are not for everyone... I suggest you give it a try if you can rent or borrow one, maybe not the 35mm f1.8... Some lenses that are really affordable in the second hand market (maybe with adapters) are the 50mm f1.7 or f1.4, you could experiment with those and see if you like the experience.

I know that advertisements can exaggerate, but I trust Nikon that VR is not totally useless at least.

I have an example of vibration reduction system. In general they perform well enough that I can use a different camera/lens combination to show the effect. In this case it is a cheap entry level Sony system camera with sensor-shift VR.

This photo was taken by my friend, a 40+ years old guy with holiday tourist kind of experience of photography and virtually no experience in cameras that weight more than half a kilogram. He only wanted a quick feel of my new camera...

First uncropped as-shot version (but resized):

Then a cropped version that also got resized to fit:

And an unresized 100% pixels of the shot:

Yes, it is very soft. My camera and lens are of very cheap entry level quality. But this shot is an example of vibration reduction, not of my camera.

Lens focal length: 150 mm (35mm equivalent: 225 mm)

Shutter speed: 1/10 second

Vibration reduction system gives you unbeatable performance on subjects that don't move. If the subject moves, then your only salvation is a fast lens. You choose.

If you really need the flexibility of zoom, consider fixed aperture versions as major upgrades over the primes. Even the cheapish (their "pro" lines which fixed aperture is are often not that cheap) sigma/tamron versions will blow the brand name kits out of the water.